const s1 = Symbol('foo')
const s2 = Symbol('foo')
console.log(s1 === s2) // false
const s3 = Symbol('lee')
console.log('s1', s1) //  Symbol(foo)
console.log('s3', s3) // s3 Symbol(lee)
console.log('没有参数的情况', Symbol() === Symbol()) // false
console.log('转化为boolean值', Boolean(s1)) // true
var obj = {}
obj[s1] = 's1'
obj[s2] = 's2'
obj.a = 'a'
console.log(obj[s1]) // s1
console.log(obj[s2]) // s2
console.log(Object.getOwnPropertySymbols(obj)) // [ Symbol(foo), Symbol(foo) ]
console.log(Reflect.ownKeys(obj)) // [ 'a', Symbol(foo), Symbol(foo) ]
